The field of the invention is surface enhancement of lanthanide chelates and applications in biological detection assays.
Surface enhancement of fluorescence is one of several phenomena derived from excitation of surface plasmons in noble metal nanoparticles in the vicinity of analyte molecules [1-2]. The growing field of plasmonics has produced a number of new techniques of great value to biomedical optics including surface plasmon resonance spectroscopy (SPRS) for reporterless detection of binding events [3-4] and surface enhanced Raman spectroscopy (SERS), which offers extremely sensitive and molecularly selective detection of analytes [5-8].
In recent years, there have been numerous publications on the development of surface-enhanced fluorescence [e.g. 9-18], and a few groups have described surface enhancement of lanthanide emission [19-22]. A 5-fold increase in the emission of europium on silver island films has been demonstrated by Weitz et al. [19]. Surface enhanced luminescence from lanthanides has also been described by Hayakawa et al. in sol gels [20]. Lakowicz et al. achieved modest increases in the emission of europium and terbium sandwiched between silver island films [21]. Nabika and Deki obtained enhanced luminescence of europium in the presence of a colloidal dispersion of silver particles [22]. All of this work has involved lanthanide ions that already have good quantum efficiency (i.e., europium and terbium) and good optical absorption (i.e., the chelate studies used an antenna to absorb the excitation light for subsequent energy transfer to the lanthanide). In this case the surface enhancement possible on the emission side is limited because the quantum efficiency cannot exceed one and the absorption is already good. The only possibility for further emission enhancement in this case is when the transition is saturated (i.e., the finite radiative or decay rate limits the ability to re-excite the fluorophore), but fluorophore saturation is uncommon. We have shown that much larger enhancements are possible for lanthanides with lower quantum efficiency or low absorption coefficient, and that these larger enhancements in turn can be used to produce larger on/off ratios for proximity assays [37].